2.3.3.1 Basic Char Syntax

Since characters are really integers, the printed representation of
a character is a decimal number. This is also a possible read syntax
for a character, but writing characters that way in Lisp programs is
not clear programming. You should always use the special read
syntax formats that Emacs Lisp provides for characters. These syntax
formats start with a question mark.

The usual read syntax for alphanumeric characters is a question mark
followed by the character; thus, ‘?A’ for the character
A, ‘?B’ for the character B, and ‘?a’ for the
character a.

For example:

?Q ⇒ 81 ?q ⇒ 113

You can use the same syntax for punctuation characters, but it is
often a good idea to add a ‘\’ so that the Emacs commands for
editing Lisp code don't get confused. For example, ‘?\(’ is the
way to write the open-paren character. If the character is ‘\’,
you must use a second ‘\’ to quote it: ‘?\\’.

You can express the characters control-g, backspace, tab, newline,
vertical tab, formfeed, space, return, del, and escape as ‘?\a’,
‘?\b’, ‘?\t’, ‘?\n’, ‘?\v’, ‘?\f’,
‘?\s’, ‘?\r’, ‘?\d’, and ‘?\e’, respectively.
(‘?\s’ followed by a dash has a different meaning—it applies
the “super” modifier to the following character.) Thus,

These sequences which start with backslash are also known as
escape sequences, because backslash plays the role of an
“escape character”; this terminology has nothing to do with the
character <ESC>. ‘\s’ is meant for use in character
constants; in string constants, just write the space.

A backslash is allowed, and harmless, preceding any character without
a special escape meaning; thus, ‘?\+’ is equivalent to ‘?+’.
There is no reason to add a backslash before most characters. However,
you should add a backslash before any of the characters
‘()\|;'`"#.,’ to avoid confusing the Emacs commands for editing
Lisp code. You can also add a backslash before whitespace characters such as
space, tab, newline and formfeed. However, it is cleaner to use one of
the easily readable escape sequences, such as ‘\t’ or ‘\s’,
instead of an actual whitespace character such as a tab or a space.
(If you do write backslash followed by a space, you should write
an extra space after the character constant to separate it from the
following text.)